Magnetic material having low hysteresis losses



Feb. 21, 1939. G. HOLST ET AL 2,147,791

MAGNETIC MATERIAL HAVING LOW HYSTERESIS LOSSES 7 Filed Nov. 22, 19340574mm 5y saw/24 mas lrweniora': GJYoZJfi, Z0. Six J1; 573061: and; Z0.G.Bwgyenr,

Patented Feb. 21, 1939 UNITED STATES PATENT OFFICE Gilles Holst, WillemSix, Jacob Louis Snoek, and

Wilhelm Gerard Burgers, Eindhoven, Netherlands, assignors to N.

V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands ApplicationNovember 22, 1934, Serial No. 754,318 In Germany December 4, 1933 8Claims.

The present invention relates to magnetic material and more particularlyto magnetic material adapted to form the core of magnetic coils,transformer coils, etc.

In many applications, for instance, for telcphone and telegraph work,transmitters, etc., and especially for so-called Pupin coils, relays,electroacoustic devices and the like, it is highly important that themagnetic material used possesses low hysteresis losses, i. e., shouldhave a magnetization curve which is substantially rectilinear.

Suitable magnetic materials for such purpose which show small hysteresislosses are certain nickel-iron alloys, possibly also containingadditions of one or more other metals as cobalt, copper or aluminum.

It has already been proposed to reduce the hysteresis losses of suchmaterials, and particularly of magnetic material consisting of anironnickel alloy, by subjecting such materials during their use toexternal compressive or tensile forces, acting on the core formed ofsuch material, either in the direction of the magnetic force or in adirection normal to said force. Thereby the crystals of the magneticmaterial are elastically deformed by such external forces, which resultsin the material exhibiting preferred directions of magnetization.

The present invention has for its purpose to provide magnetic materialsexhibiting definite preferred directions of magnetization, whichmaterials can be obtained in a simple manner and without the applicationof external mechanical tensile or compressive stress in the direction ofthe magnetic force or normally thereto.

According to the invention, the magnetic material employed isanisotropic and, when used as a core, the direction of the magnetizingforce is so selected as to extend normally or substantially normally tothe direction of the greatest permeability of the material.

The following explanation will give a better understanding of thepresent invention.

,It is known that a ferromagnetic body which consists of asingle-crystal, entirely free from strains, exhibits three preferreddirections of magnetization which, in cube-shaped space-grids, coincidewith the main axes of the cube.

Assuming a Cartesian co-ordinate system established in such a singlecrystal with the X, Y and Z directions coinciding with the three mainaxes of the cubes; the single crystal will be equally well magnetizablein the positive and in the negative X, Y and Z directions.

Furthermore,.a single crystal comprises, in the same manner as any otherferromagnetic body, a certain number of areas (so-called Weisscomplexes), within each of which areas the internal or molecular fieldhas the same orientation throughout, These directions of spontaneousmagnetization of the Weiss complexes coincide with the cube "edges andcan be considered to lie in the positive and negative X, Y and Zdirections. Since the probability that the direction of the spontaneousmagnetization coincides with the positive or negative directions is thesame, the action of the Weiss complexes neutralize each other towardsthe outside.

Thus, if a magnetic field is set up from the outside, the direction ofthe spontaneous magnetization orients itself in the direction of themagnetic field and this orientation, in the case of an idealsingle-crystal, occurs without any hysteresis. For an explanation ofthis phenomenon the so-called reversible wall displacement andreversible rotation hypotheses have been offered.

As has been stated above, the preferred directions of magnetization in asingle-crystal iron coincide with the main axes of the cube-shapedspace-grid. In an ideal single-crystal an infinitely small externalfield, applied in the direction of one of the main axes, will besufficient to cause the vector of the spontaneous magnetization to havethe same direction as the field, so that the permeability would have analmost infinitely large value. However, the artificially-produced singlecrystals available in practice differ from the ideal single-crystal, dueto the impurities always contained in same, whereby a magnetic field offinite strength is required for magnetic saturation.

. Even with such impurities, however, the artificially manufactured ironsingle-crystals still exhibit a very high initial permeability in thepreferred directions, and, despite the impurities contained therein,hysteresis losses of these single-crystals are but small.

In view of the above, it appears that a ferromagnetic body consisting ofa single crystal would be the ideal core material for choke coils,transmitters and the like. However, in many devices, such as forinstance, Pupin coils, a too high initial permeability is undesirable.While the effective permeability of a magnetic core consisting of asingle crystal could be reduced by providing air-gaps in the core, sucha measure brings about various inherent drawbacks and diificulties. Oneof the drawbacks of coils having a core with airgaps is that thereexists an external field so that undesirable induction effects on othercoils in the neighbourhood will occur.

The present invention has for its purpose to obtain a magnetic materialof low hysteresis losses without at the same time obtaining undulyhighinitial permeability, and is based on the realization that, byartificially suppressing one of the preferred directions of themagnetization-of a single-crystal, it is possible to also decrease thepermeability in the suppressed direction.

As will be explained in more detail hereinafter, this can be eifected bymeans of a suitable mechanical and thermal treatment by which theferromagnetic material, for instance an ironnickel alloy, is firstbrought into a state in which the structure corresponds substantially tothat of a single-crystal. Subsequently the material is subjected tofurther mechanical treatment to obtain the desired magnetic anisotropyin which the material exhibits, instead of three, only two preferreddirections of magnetization. The anisotropic property so exhibited isintimately associated with the state of the internal stress of thematerial brought about by the above treatments.

In order that the invention will be clearly understood and more readilycarried into eflect it will be more fully explained with reference tothe accompanying drawing, in which,

Figure 1 is a graph showing magnetization curves, and

Fig. 2 is a perspective view of a loading coil.

Below will be given a specific example to obtain a magnetic material. inaccordance with the invention:

Iron and nickel are fused together in a furnace in the proportion ofabout 50% of iron and 50% of nickel. The molten composition is formed ina mould and cooled to'form a bar or rod which is formed at a temperaturebetween 1000-1100 C. into a bar having a square cross-section whosesides have a length of 5-8 cm. The latter is rolled at a temperaturebetween 900-1100 C. into a tape of 1-4 mm. thickness, which tape isannealed in hydrogen at a temperature between IOU-900 C. According tothe invention this material of a thickness of about 1 mm. is thensubjected to repeated rolling without annealing above therecrystallization temperature to decrease its thickness to about 110microns so that the crosssection of the tape is reduced or still more.Now the band or tape is subjected to recrystallization by annealing itat about 1100 C. As a result chiefly of the reduction in cross sectionof 90% or more and also of the recrystallization, the examination of theband by X-rays shows that the structure of the band closely correspondsto that of a single crystal. In correspondence therewith the magneticexamination shows two pronounced preferred directions of magnetization;one falls in the direction of rolling, i. e. in the longitudinaldirection of the band, and the other at right angle thereto, namely, inthe direction of the width of the band. Athird preferred direction ofmagnetization normally to both these directions or to the surface of thehand must also be present; however because of the small thickness of theband, determination thereof entails considerable difiiculties. Theinitial permeability of the band in the longitudinal and width directionis about 500. The recrystallized band is subjected to cold rolling todecrease its thickness in one or more rolling steps to about 60 microns.This causes a considerable decrease in the permeability of the materialin direction of rolling, l. e. in the length direction of the band,without, however, materially affecting the permeability in the directionof the width of the band. It follows that the band is renderedanisotropic. in which the ordinate axis represents the magnetization (I)and the abscissa axis represents the field strength (H). Curve Irepresents the magnetization of the band taken in its width directionand curve II represents the magnetization of the band taken in itsrolling or longitudinal direction. From. these curves it will be seenthat the permeability in the longitudinal direction is only a fractionof permeability in the width direction and is nearly constant in a widerange of field strength H. The decrease in permeability in thelongitudinal or rolling direction is accompanied by a small increase ofthe hysteresis losses, which, however, remain within permissible limits.The following data will be of interest.

Longitudinal direction direction Coercive force 8 l. 5 Remanentmagnetisation 30 950 Initial permeability 40 400 Maximum permeability 504000 Hysteresis value q 7-8 7-8 The above value was measured with analternating current of 1 milliampere at a frequency of 800 cycles for acoil provided with a core having a volume of 20 c. c. As above stated,the rolling of the recrystallized band from microns down to 60 micronsonly decreases the permeability in the rolling or longitudinal directionof the band, and does not materially afiect the permeability in thewidth direction. Although it is difiicult to determine the permeabilityin the direction normal to the surface of the band, it appears that thispermeability also remains substantially unaffected. If it is desired toincrease the permeability in the longitudinal direction of the 60 micronband, this can be effected by a thermal treatment at a temperature ofabout 400 0. Such thermal treatment partly removes the internal stressesand causes an increase in the permeability in.the rolling direction. Inthe case of the above described material, the initial permeability wasraised from 40 to 80 by a thermal treatment at about 420? C. for twohours without, however, substantially changing the hysteresis losses.The improved magnetic material according to our invention is especiallyuseful for loading coils. For this purpose a band of the magneticmaterial is formed and spirally wound into a core upon which thewindings of the loading coil are mounted. Such a coil is shown in Fig. 2in which the core 2 is formed of a spirally wound band whose roiling orlongitudinal direction-in which the permeability is a minimum-isindicated by the arrow. A winding l-I is provided on the core 2. Flow ofcurrent through the The above is illustrated in Fig. 1,

winding l| sets up in the core 2 a magnetic field whose direction isindicated by the arrow and coincides with the direction of minimumpermeability of the core. Although we have described our invention inconnection with specific examples, we do not wish to be limited'thereto. The same results are obtained with nickel iron alloys of othercomposition than 50% of iron and 50% of nickel. Such alloys may alsoinclude other metals such as cobalt, copper, and aluminium, which can beadded without disturbing the regular orientation of the nickel ironcrystals.

What we claim is:

1. The process of providing a magnetic material of low hysteresis lossescomprising the steps of cold rolling a band consisting of substantiallyequal parts of nickel and iron from a thickness of about 1 mm. to athickness of about 110 microns, recrystallizing said band by heattreatment at a temperature of abolt 1100 C., and cold rolling said banddown to microns.

2. The process of providing a magnetic material of low hysteresis lossescomprising the steps of coldrolling a band consisting of substantiallyequal parts of nickel and iron from a thickness of about 1 mm. to athickness of about 110 microns, recrystallizing said band by heattreatment at a temperature of about 1100 0., cold rolling said band downto 60 microns,and partly removing the internal stresses from said bandby heat treatment at a temperature of about 400 C.

3. The process of producing a. magnetic material of substantiallyconstant permeability and low hysteresis losses comprising the steps of,cold-rolling a band of a nickel-iron alloy in the middle percentagerange to reduce its thickness at least 90%, heat-treating the band abovethe recrystallizing temperature, said cold-rolling and heat-treatingbringing most of the crystals into substantially the same orientationand producing a preferred direction of magnetization coinciding with therolling direction, and subsequently cold- Y rolling said band tointernally stress the same and to suppress said preferred direction ofmagnetization.

4. The process of producing a. magnetic material of substantiallyconstant permeability and low hysteresis losses comprising the steps of,coldrolling a nickel-iron alloy band of substantially equal parts ofnickel and iron to reduce its thickness at least 90%, heat-treating theband above the recrystallizing temperature of the alloy, saidcold-rolling and heat-treating bringing most of the crystals intosubstantially the same orientation with two cube edges lying in theband-plane, one edge extending in the rolling direction and the otheredge normal thereto, and subsequently cold-rolling the band to suppressthe preferred direction of magnetization extending in the rollingdirection.

5. The process of producing a magnetic material of substantiallyconstant permeability and low hysteresis losses comprising the steps of,cold-rolling a band of nickel-iron alloy in the middle percentage rangesto reduce its thickness at least 90%, heat-treating the band above therccrystallizing temperature, said cold-rolling and heating bringing mostof the crystals into substantially the same orientation and producing apreferred direction of magnetization coinciding with the rollingdirection, cold-rolling the band to internally stress the same and tosuppress said preferred direction of magnetization, and heattreating theband to partly remove the internal stresses.

6. The process of producing a. magnetic material of substantiallyconstant permeability and low hysteresis losses comprising the steps of,coldrolling a nickel-iron alloy band of substantially equal parts ofnickel and iron to reduce its thickness at least 90%, heat-treating theband above the recrystallizing temperature of the alloy, saidcold-rolling and heat-treating bringing most of the crystals intosubstantially the same orientation with two cube edges of each crystallying in the band plane, one edge extending in the direction of rollingand other edge normal thereto,

cold-rolling the band to internally stress the same and to suppress thepreferred direction of magnetization extending in the rolling direction,and heat-treating the band to remove part of the internal stresses.

'7. The process of producing a. homogeneous magnetic material ofsubstantially constant permeability and low hysteresis losses,comprising the steps of cold-working without intermediate annealing aband of a nickel-iron alloy in the middle percentage range to reduce itsthickness at least of the order of 90%, heat-treating the material abovethe recrystallizing temperature, said cold-working and heat-treatingbringing most of the crystals into substantially the same orientationand producing preferred directions of magnetization, and subsequentlycold-working the material to internally stress the same and to suppressone of the preferred directions of magnetization.

8. A homogeneous magnetic-core material having substantially constantpermeability and low hysteresis losses produced in accordance with theprocess specified in claim '7.

GILES HOLST. WILLEM SIX.

. JACOB LOUIS SNOEK.

WIm-IEIM GERARD BURGERS.

